Fiber lasers are of great current interest for high-power laser applications. Many of these applications utilize the coherence of the laser light and thus require controlled polarization and narrow linewidth output radiation, such as for coherent LIDAR, frequency conversion or beam combining via spectral or coherent combining techniques. The narrow linewidth and polarized output can limit power scaling of the fiber laser output due to a number of effects.
The limitations of fiber lasers include nonlinearities (e.g., Stimulated Brillouin Scattering (SBS)) and Modal Instabilities (MI). For example, in some cases, modal instabilities in Large-Mode-Area (LMA) high-power fiber amplifiers limit power scaling from individual fibers. LMA fibers typically support several transverse modes, but are preferentially operated with the majority of the light in the fundamental (LP01) mode to optimize beam quality. The modal instability can transfer power out of the LP01 mode to a higher order mode, degrading beam quality. In some cases, this transfer to higher order mode(s) may result in light being coupled out of the core into the fiber cladding, limiting output power. With regard to nonlinearities, some of the techniques used to avoid SBS (e.g., larger core size and reduced fiber length) may result in MI.
Papers and presentations reporting on modal instabilities in high power fiber amplifiers include A. Smith, J. J. Smith, “Mode Instability in High Power Fiber Amplifiers,” Optics Express 19, 10180-10192 (2011) (hereinafter, “A. Smith et al.”); C. Jauregui, T. Eidam, J. Limpert, and A. Tünnermann, “The impact of modal interference on the beam quality of high-power amplifiers,” Opt. Express 19, 3258-3271 (2011) (hereinafter, “C. Jauregui et al.”); and J. Edgecumbe, Nufern, “Single Mode, High Power, Narrow Line-width Fiber Amplifiers,” 24th Solid State and Diode Laser Technology Review (2011) (hereinafter, “J. Edgecumbe”), each of which is incorporated herein by reference in its entirety. Recent reports in the literature such as the papers identified above indicate a possible thermal origin for modal instabilities (e.g., thermal load per unit length of the fiber) and the likely role of induced gratings by interactions between fundamental mode and higher order mode content.
Teemu Kooki et al., “Fiber amplifier utilizing an Yb-doped large-mode-area fiber with confined doping and tailored refractive index profile,” Proc. SPIE 7580, Fiber Lasers VII: Technology, Systems, and Applications, 758016 (Feb. 17, 2010) (hereinafter, “Kooki et al.”) is incorporated herein by reference in its entirety. Kooki et al. describe power scaling of Yb-doped large-mode-area fibers drives the scaling of the mode area in order to suppress nonlinearities. Two Yb-doped large-mode-area fibers were manufactured using the Direct Nanoparticle Deposition process: one with a step refractive index profile and active ion confinement, and another with a tailored refractive index and active ion confinement. The index tailoring and doping profiles were designed based on literature to enhance the beam quality of the fibers. Both fibers exhibited a mode field diameter comparable to a 40 μm step index fiber with 0.07 NA. The fibers were characterized for their geometries, index profiles, and material composition profiles. Additional testing for beam quality and nonlinearities in pulsed operation was conducted using a power amplifier setup. The beam quality enhancement capability of the tested fibers was inconclusive due to incomparable launching conditions of the signal to the fibers.
U.S. Pat. No. 4,829,529 to James D. Kafka (hereinafter, “Kafka”) titled “LASER DIODE PUMPED FIBER LASERS WITH PUMP CAVITY”, issued May 9, 1989, and is incorporated herein by reference in its entirety. Kafka describe a fiber laser having a single mode fiber core of laser material is pumped by a high power coherent laser diode source by providing a multi-mode fiber around the single mode core to define a pump cavity which propagates pump radiation while allowing the pump radiation to couple to the single mode core. Laser diode arrays and extended emitter laser diodes can be used to pump a single mode fiber by inputting the pump radiation into the multi-mode fiber surrounding the single mode fiber core. The multi-mode [sic] fiber has a much greater diameter than the single mode core.
U.S. Pat. No. 5,508,842 to Keiko Takeda et al. (hereinafter, “Takeda et al.”) titled “OPTICAL AMPLIFIER”, issued Apr. 16, 1996, and is incorporated herein by reference in its entirety. Takeda et al. describe an optical amplifier for amplifying a signal light by propagating the signal light and a pumping light in a rare earth element doped fiber doped with a rare earth element. A diameter of a rare earth element doped portion of the rare earth element doped fiber is gradually reduced in a direction of propagation of the pumping light. With this construction, an adverse rare earth element doped area which does not contribute to optical amplification, but rather attenuates the pumping light, can be eliminated to thereby provide an optical amplifier having increased amplification efficiency.
U.S. Pat. No. 5,708,669 to David John DiGiovanni et al. (hereinafter, “DiGiovanni et al. '669”) titled “ARTICLE COMPRISING A CLADDING-PUMPED OPTICAL FIBER LASER”, issued Jan. 13, 1998, and is incorporated herein by reference in its entirety. DiGiovanni et al. '669 describe a cladding pumped optical fiber laser comprises a length of optical fiber having a rare earth-doped region of diameter dRE>d01 where d01 is the mode diameter of the LP01 mode of the fiber at the laser radiation at wavelength λ. In one embodiment the fiber has a core diameter dc selected such that the LP01 mode is the only guided spatial mode of the fiber, and dRE is greater than dc. In another embodiment the fiber supports at least one higher order guided spatial mode, typically LP11 or LP02, and dRE is approximately equal to or larger than dc. Currently preferred embodiments comprise a grating-defined laser cavity that comprises a mode-coupling refractive index grating. Cladding pumped lasers according to the invention will typically have efficient conversion of pump radiation to laser radiation, and consequently can typically be shorter than analogous prior art cladding pumped lasers.
U.S. Pat. No. 5,864,644 to David John DiGiovanni et al. (hereinafter, “DiGiovanni et al. '644”) titled “TAPERED FIBER BUNDLES FOR COUPLING LIGHT INTO AND OUT OF CLADDING-PUMPED FIBER DEVICES”, issued Jan. 26, 1999, and is incorporated herein by reference in its entirety. DiGiovanni et al. '644 describe light coupled from a plurality of semiconductor emitters to a cladding-pumped fiber via tapered fiber bundles fusion spliced to the cladding-pumped fiber. Individual semiconductor broad stripe emitters can be coupled to individual multimode fibers. The individual fibers can be bundled together in a close-packed formation, heated to melting temperature, drawn into a taper and then fusion spliced to the cladding-pumped fiber. The taper is then overcoated with cladding material such as low index polymer. In addition, a fiber containing a single-mode core can be included in the fiber bundle. This single-mode core can be used to couple light into or out of the single-mode core of the cladding-pumped fiber.
U.S. Pat. No. 6,289,027 to Brian L. Lawrence et al. (hereinafter, “Lawrence et al.”) titled “FIBER OPTIC LASERS EMPLOYING FIBER OPTIC AMPLIFIERS”, issued Sep. 11, 2001, and is incorporated herein by reference in its entirety. Lawrence et al. describe ring and linear cavity, fiber optic laser systems employing non-invasive fiber optic amplification technology. A channel overlay waveguide is employed for amplification of optical energy evanescently coupled to the overlay waveguide from the fiber optic. One of two amplification methods can be employed. The first involves inducing stimulated emission with the overlay waveguide and the second uses a second order, non-linear frequency conversion to down-convert a high-power, short-wavelength pump signal into the waveguide to amplify the optical energy coupled thereto. Amplification of optical energy in the channel overlay waveguide can be established within a single beat length of evanescent removal to evanescent return of the optical energy to the fiber optic. Intra-cavity elements can be employed to effect, e.g., wavelength selection, optical isolation, or modulation of the resultant, optical signal propagating in the fiber optic.
U.S. Pat. No. 6,324,326 to Matthew J. Dejneka et al. (hereinafter, “Dejneka et al.”) titled “TAPERED FIBER LASER”, issued Nov. 27, 2001, and is incorporated herein by reference in its entirety. Dejneka et al. describe a tapered fiber laser having a multi-mode section, a single-mode section, and either a tapered section or fundamental mode matching junction therebetween. The multi-mode section has a large core to directly receive pump light from a broad stripe laser or diode bar, and a length preferably longer than the absorption length of the pump light (so optical amplification occurs predominantly in the multi-mode section). Doping levels can be increased to reduce the multi-mode length. The taper angle is sufficiently small to produce adiabatic compression of the fundamental mode from the multi-mode to single-mode sections, and acts as a cutoff filter favoring lasing of the fundamental mode within the multi-mode section. Alternately, the step junction may have a mode field diameter matched to the lowest-order mode, with laser light output via the single-mode section. The invention can be applied to waveguides (particularly those having an aspect ratio corresponding to a broad stripe laser source), doped with ytterbium or neodymium ions, and is particularly advantageous as a pump source for an erbium-doped fiber amplifier (EDFA).
U.S. Pat. No. 6,970,624 to David J. DiGiovanni et al. (hereinafter, “DiGiovanni et al. '624”) titled “CLADDING PUMPED OPTICAL FIBER GAIN DEVICES”, issued Nov. 29, 2005, and is incorporated herein by reference in its entirety. DiGiovanni et al. '624 describe optical fiber gain devices, such as lasers and amplifiers, wherein losses due to a large step transition between an input section and a gain section are reduced by inserting an adiabatic transformer between the input section and the gain section. In the preferred case the adiabatic transformer comprises a GRadient INdex (GRIN) lens. The lens serves as an adiabatic beam expander (reducer) to controllably increase (reduce) the modefield of the beam as it travels through the step transition.
U.S. Pat. No. 7,557,986 to Yoav Sintov (hereinafter, “Sintov”) titled “HIGH POWER FIBER AMPLIFIER”, issued Jul. 7, 2009, and is incorporated herein by reference in its entirety. Sintov describes a high power fiber amplifier including a double clad fiber including a protective outer jacket (41), an outer clad (44), an inner clad (42, 35) and a doped core (43, 34, 32), and a source of pump power coupled to the inner clad through coupling optics (22) and at least one of a side-fiber coupling section and an end-fiber coupling section, wherein the inner clad includes a large diameter core portion (34), operative as a high power amplification stage, capable of absorbing the majority of the pump power, and a small diameter core portion (32), operative as a low power amplification stage, wherein both core portions, pumped through the inner clad (35), are serially connected through an optical interface point (37).
U.S. Pat. No. 7,809,236 to Martin H. Muendel (hereinafter, “Muendel”) titled “OPTICAL FIBER HOLDER AND HEAT SINK”, issued Oct. 5, 2010, and is incorporated herein by reference in its entirety. Muendel describes an optical fiber holding device having an optical fiber held therein. The device has a base with a spiral channel in an upper surface holding and housing the optical fiber. The channel has a first location where the fiber enters leading to a plurality of turnings for holding the optical fiber wrapped there-around at another end a second location where the fiber exits the channel wherein the bend radius of the optical fiber housed within the spiral channel is at least 2 cm. The dimensions are such that housing forms a heat sink allowing heat within the fiber to dissipate within the base. The spiral channel is preferably designed to keep the fiber within the channel and to prevent it from inadvertently springing out spring tension of the bent fiber holds the fiber within the groove or channel.
Accordingly, there is a need in the art for improved fiber amplifier systems that suppress modal instabilities.